ARM: dts: imx25-pdk: Provide an Ethernet PHY reset

GPIO4_8 is connected to the Ethernet PHY reset line, so populate the
'phy-reset-gpios' property accordingly.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>